ROGERS, GEOLOGY OF THE CORTLAXDT SERIES- 19 



extension is complete in itself, as is that of Hobbs on the Connecticut 

 areas ; but as for the main area, the most recent work done on it— that of 

 Berkey on the boundaries — is what should have been the earliest. Owing 

 to this somewhat unfortunate reversal, there are several erroneous concep- 

 tions extant concerning the series; and the writer, while not pretending 

 to the experience of Dana or the erudition of Williams, hopes in the fol- 

 lowing pages to give a correct general notion of the rocks as a whole, leav- 

 ing to later investigators that part of the more specialized work which is 

 yet undone. 



Correlation 



The correlation of the series is, like that of many such igneous masses, 

 open to considerable speculation. Evidence of its age with respect to the 

 surrounding rocks is indubitable ; in a number of places, marked contact 

 action is shown, and on Verplanck's Point especially a number of igneous 

 dikes are intruded into the schist and limestone, so that the Cortlandt 

 series is unquestionably younger. The lower limit, then, is dependent on 

 the age of the Manhattan schist and the associated Inwood limestone. 



There is, however, considerable difference of opinion as regards the 

 correlation of these rocks. A. C. Spencer 18 and his associates regard them 

 as the metamorphosed equivalents of the Cambrian and Ordovician, which 

 occur in an unaltered condition north of the Highlands. C. P. Berkey, 19 

 on the other hand, who has been mapping the Tarrytown and West Point 

 Quadrangles in the Highlands of New York, finds that the Manhattan 

 schist and Inwood limestone are separated by unconformities from both 

 the basement gneiss series below and the Poughquag quartzite above, this 

 latter representing the base of the Paleozoic. The schist and limestone 

 are therefore pre-Cambrian. 



The Cortlandt Series has been intruded into these rocks and is there- 

 fore at least later than pre-Cambrian time. Moreover, since the schist 

 and limestone were strongly metamorphosed, probably in the Green Moun- 

 tain upheaval, the series must be post-Ordovician. There is no criterion, 

 however, by which we can fix the upper limit. Van Hise and Leith 20 

 make the following rather cryptic remark: "The rocks of the Cortlandt 

 Series (the elastics) of the original Taconic area and of the upper series 

 of the Adirondacks are of the same age, t. e., Taconic, or Lower Cam- 



18 A. C. Spencer summarizes his own work and that of W. S. Bajiey in New Jersey, 

 and the work of E. C. Eckel and F. J. H. Merrill in New York, in Bulletin 360, U. S. 

 G. S., "Precambrian Geology of North America.'' by Van Hise and Leith, p. G34. 1909. 



w Op. cit., pp. 361-378. 



20 Op. cit., p. 319. 



